1. Field of the Invention
The present invention relates to an optical system evaluation apparatus, which evaluates optical characteristics of an optical system to be evaluated on the basis of an image forming position of a point image formed through the optical system and an optical system evaluation method.
2. Description of the Related Art
In general, when an image imaged by using an optical system is used for length measurement, the optical system is required to have high telecentricity. The telecentricity represents a state in which all main beams at the object side or an image side are parallel to an optical axis, regardless of the distance from the optical axis. By way of example, an object-side telecentric optical system is used in a length measuring microscope. In this case, even if an object to be imaged is moved away from the focus position, the image forming position corresponding to an arbitrary point on the object is not changed. Thus, it is possible to measure the distance between two points on the object with high accuracy without depending on the focusing state or a placement position of the object.
In the prior art, as the evaluation apparatus for an optical system, there has been known especially the evaluation apparatus which evaluates resolution of lens (e.g., see Japanese Patent Application Laid-Open No. 2004-163207). In the evaluation apparatus described in the patent document, an aerial image formed by a lens to be examined is imaged, and the MTF of the lens to be examined is calculated on the basis of the information of the imaged image. This method for evaluating the optical characteristics of the optical system by observing the aerial image formed by the optical system such as lens is well-known, and the technique for evaluating the telecentricity of the optical system by using this method is widely known. Here, the prior art for evaluating the telecentricity of the optical system is described with reference to FIG. 11. FIG. 11 shows an example of evaluating an image-side telecentric optical system.
As shown in FIG. 11, in the prior art optical system evaluation apparatus, a pin-hole 71 as a point light source is provided on an object surface of an optical system 72 to be evaluated, and is illuminated from behind by an illumination means (not shown). A point image 73 as the aerial image of the pin-hole 71 is formed on the image surface of the optical system 72 to be evaluated. However, since this point image 73 is small, it is difficult to measure the image forming position with the maximum intensity with high accuracy even if the point image 73 is directly imaged by an imaging device. Therefore, the image enlarged by an enlargement optical system 74 is imaged by the imaging device 75. Thus, the image forming position of the point image 73 can be determined by retrieving a pixel with the maximum brightness from the image region in which the enlarged image is imaged.
Further, images are sequentially imaged while moving the enlargement optical system 74 and the imaging device 75 in the optical axis direction (Z axis direction) of the optical system 72 to be evaluated and a pixel with the maximum brightness is retrieved from each image sequentially imaged, whereby it is possible to detect the lateral shift of the X and Y coordinates of the image forming position accompanying the change of the Z coordinates. Further, the tendency of the change of the lateral shift with respect to the Z coordinates is calculated, whereby the telecentricity corresponding to the point image 73 can be evaluated. Moreover, this measurement is applied to a plurality of the point images 73, whereby the distribution of image-side telecentricity of the optical system 72 to be evaluated can be evaluated.
However, in order to calculate the X, Y and Z coordinates of the image forming position of the point image 73, the respective positions of the enlargement optical system 74 and the imaging device 75 should be monitored by a triaxial measuring instrument. Meanwhile, in order to evaluate the distribution of the telecentricity in the entire view field of the optical system 72 to be evaluated, the image should be measured while moving the pin-hole 71 to a plurality of positions on the object surface. In this case, the X and Y coordinates of the pin-hole 71 should also be monitored by the length measuring instrument.
There has been known that the coma aberration influences the tendency of the change of the lateral shift of the image forming position with respect to the Z coordinates. When there is no coma aberration, the lateral shift of the image forming position is linearly changed with respect to the Z coordinates, while as the coma aberration is increased, the lateral shift is changed in a curved line.